Beam delivery system for molecular fluorine (F2) laser

ABSTRACT

A system is provided for delivering a laser beam of wavelength less than 200 nm from a laser, such as an F 2  laser or ArF laser, through a sealed enclosure sealably connected to the laser, and preferably to another housing, leading ultimately to a workpiece. The enclosure is preferably evacuated and back-filled with an inert gas to adequately deplete any air, water, hydrocarbons or oxygen within the enclosure. Thereafter or alternatively, an inert gas flow is established and maintained within the enclosure during operation of the laser. The inert gas preferably has high purity, e.g., more than 99.5% and preferably more than 99.999%, wherein the inert is preferably nitrogen or a noble gas. The enclosure is preferably sealed by a window transparent to the sub-200 nm radiation for preventing contaminants generated in the enclosure from entering the housing and contaminating surfaces therein. The enclosure is preferably made of steel and/or copper, and the window is preferably made of CaF 2 .

PRIORITY

This Application is a divisional application which claims the benefit ofpriority under 37 C.F.R. 1.53(b) to U.S. patent application Ser. No.09/594,892, filed Jun. 14, 2000, which is continuation-in-part of U.S.patent application Ser. No. 09/343,333, filed Jun. 30, 1999, now U.S.Pat. No. 6,219,368 which claims the benefit of priority to U.S.Provisional Patent Application No. 60/119,973, filed Feb. 12, 1999, eachof which is hereby incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a beam delivery system for use withlasers, and particularly for use with discharge pumped molecularfluorine lasers emitting around 157 nm.

2. Discussion of the Related Art

Molecular fluorine (F₂) lasers operating at a wavelength ofapproximately 157 nm are a likely choice for deep UV/ vacuum UVmicrolithography with resolution below 0.1 micrometer. Laser radiationat this wavelength is also very useful for micromachining applicationsinvolving materials normally transparent at commonly available laserwavelengths.

Efficient extracavity transport of a sub-200 nm laser beam to the targetis complicated by strong absorption in the atmosphere. That is, thesub-200 nm laser beam of such a laser will propagate a certain distancealong an extracavity beam path between the laser output coupler and awork piece where it is subject to absorptive losses due to anyphotoabsorbing species such as water, oxygen and hydrocarbons locatedalong the beam path. For example, an extinction length (1/e) for 157 nmradiation emitted by the F₂-laser is less than a millimeter in ambientair.

High intracavity losses also occur for lasers operating at wavelengthsbelow 200 nm, again due particularly to characteristic absorption byoxygen and water, but also due to scattering in gases and all opticalelements. As with the absorption, the short wavelength (less than 200nm) is responsible for high scattering losses due to the wavelengthdependence of the photon scattering cross section.

These complications from absorption and scattering are much less of aproblem for conventional lithography systems employing 248 nm light,such as is emitted by the KrF-excimer laser. Species such as oxygen andwater in the cavity and atmosphere which absorb strongly below 200 nm,and specifically very strongly around 157 nm for the F₂ laser, exhibitnegligible absorption at 248 nm. The extinction length in ambient airfor 248 nm light is substantially more than ten meters. Also, photonscattering in gases and optical elements is reduced at 248 nm comparedwith that occurring at shorter wavelengths. In addition, output beamcharacteristics are more sensitive to temperature-induced variationseffecting the production of smaller structures lithographically at shortwavelengths such as 157 nm, than those for longer wavelength lithographyat 248 nm. Clearly, KrF excimer lasers do not have the same level ofproblems since the 248 nm light scatters less and experiences lessabsorption.

One possible solution for dealing with the absorption problems of the157 nm emission of the F₂ laser is sealing the beam path with a housingor enclosure and purging the beam path with an inert gas. However, highflow rates are typically used in this technique in order to minimize thedown time needed to remove absorbing species from the beam enclosure.That is, starting from a state where the enclosure is filled withambient air, an unacceptably long purge time and high flow rate would berequired to bring the partial pressure of absorbing species down to areasonable level. It may also be necessary to perform this purgingtechnique with a very clean inert gas, e.g., containing less than 1 ppmof absorbing species such as water and oxygen. Commercial ultra highpurity (UHP) grade gases may be obtained to satisfy these purityrequirements at increased cost. Overall, this purging approach isexpensive and inconvenient.

Another solution would be evacuating the beam path. In this case, arelatively low pressure vacuum would be needed resulting in an expensivepumping system. For example, ultrahigh vacuum (UHV) pumping equipmentand techniques may be necessary for achieving a pressure below 100millitorr. Such equipment and techniques combine a tight enclosure withhigh pumping capacity. Unsatisfactorily long initial pumping times wouldstill be required. In this evacuation approach, transmission along theoptical beam path enclosure would be determined by the absorption ofradiation by “residual” gases, mainly oxygen, water vapor andhydrocarbons which remain despite the evacuation, e.g., particularlyattached to the interior walls of the enclosure.

FIG. 1 shows an experimentally measured dependence of the transmissionof a 0.5 meter optical path on the residual air pressure. A theoreticalfit is also shown in FIG. 1 and is based on the assumption that the mainabsorbing species is water vapor having an absorption cross-section ofapproximately 3×10⁻¹⁸ cm². This assumption is believed to be justifiedbecause water has a tendency to be adsorbed at the walls of vacuumsystems and thus, to dominate the residual pressure in such systems.

As can be seen, at a residual pressure of 50 milliTorr, the opticallosses amount to about 1% per each 0.5 meter of the optical path. Ataround 100 milliTorr, the optical losses amount to about 2% per each 0.5meter. At 150 milliTorr and 200 milliTorr, respectively, the lossesamount to 3% and 4.5%. In a system such as a microlithographic stepper,the optical beam path can be as large as several meters which would leadto an unsatisfactorily high total amount of losses at that loss rate.For example, an average five meter beam path, even at a transmittancebetween 99% and 95.5%, as shown for 50-200 milliTorr residual pressuresin FIG. 1, corresponds to between a 10% and 37% loss.

Another consideration is the energy stability. It is desired to maintainlaser energy dose variations, and/or energy moving average variations,to less than, e.g., 0.5%. If residual oxygen or water vapor partialpressures fluctuate by 0.5% to 1.0%, e.g., then fluctuations in theabsorption of the beam by these species could cause the energy dosestability to fall below desired or even tolerable levels. It isrecognized in the present invention that a first step of lowering thepartial pressures of photoabsorbing species along the laser beam pathwould serve to lower the % absorption fluctuation and increase theenergy dose stability, even if the % concentrations of these speciesfluctuate at the same % value. It is desired, then, to have a techniquefor preparing the beam path of a VUV laser such that absorption andabsorption fluctuations of the beam along the beam path are low enoughto meet energy dose stability criteria, e.g., of <0.5%.

It is clear from the above measurement and theoretical fit for the beampath evacuation technique that one needs to lower the residual pressureof the absorbing species substantially below 100 milliTorr to achieveacceptable optical losses, e.g. less than around 1% per meter of opticalpath length, and acceptable optical loss fluctuations. Such lowpressures can only be obtained using complex and expensive vacuumequipment and/or operating the vacuum equipment for an unacceptably longtime. All together, this leads to a substantial and undesirable downtimefor pumping and requires complex and expensive equipment. An approach isneeded for depleting the beam path of a laser operating below 200 nm,particularly an F₂ laser, of photoabsorbing species without incurringexcessive down times or costs.

It is recognized in the present invention that photoabsorbing speciesmay tend to accumulate in greater concentrations along a beam path of asub-200 nm laser beam than would otherwise accumulate along a similarlength, e.g., of an enclosure otherwise substantially free ofphotoabsorbing and/or other contaminant species. This contaminationgeneration has been observed experimentally to occur along the beam pathfrom the VUV laser to an imaging system, workpiece, or other externalapplication process equipment. It is desired that such photoabsorbingand/or other contaminant species be prevented from exiting the enclosureand contaminating another environment, such as a housing connected tothe enclosure which may contain an imaging system and/or workpiece.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a laser systemwherein a beam path of the laser beam exiting the laser is substantiallydepleted of species which photoabsorb strongly below 200 nm includingsuch species as air, water, oxygen and hydrocarbons.

It is a further object to provide a system wherein contaminantsgenerated along a beam path of the laser beam exiting the laser areflushed from the beam path and/or prevented from crossing from the beampath into an external enclosure, while the beam is allowed to propagateinto the external enclosure.

In accordance with the above objects, a beam delivery system forconnecting to a laser emitting a laser beam at less than 200 nm and fordelivering the laser beam to an external housing leading ultimately to aworkpiece is provided. The system includes an enclosure sealing at leasta portion of the beam path exiting the laser from the outer atmosphere,the enclosure includes a plurality of ports for flowing an inert gas, ofpreferably 99.5% purity or more, within the enclosure to enable thelaser beam to propagate along the beam path, such that the energy of thebeam can traverse enclosure without substantial attenuation due to thepresence of photoabsorbing species along the beam path. A windowpreferably seals the enclosure that is substantially transparent at theemission wavelength of less than 200 nm to allow the beam to exit theenclosure and enter the external housing, while preventing contaminantsgenerated within the enclosure from exiting the enclosure andcontaminating surfaces within the housing.

Propagation with significant transmittance of the 157 nm emission of amolecular fluorine (F₂) laser along the beam path is specificallyenabled in the present invention, as well as for ArF, Xe, Kr, Ar, and H₂lasers operating respectively at 193 nm, 172 nm, 145 nm, 125 nm and 121nm. Absorption and absorption fluctuations are advantageously maintainedat a low level within the enclosure for greater efficiency, energystability and energy dose stability. The sub-200 nm beam is allowed topropagate along the beam path within the enclosure, and then to exit theenclosure, preferably into a second enclosure such as may include anoptical imaging system of a photolithography system, leading ultimatelyto a workpiece, while contaminants generated within the enclosure areprevented from exiting the enclosure due to the presence of the windowsealing the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a dependence of the transmittance of a 157 nm beampropagating along a 0.5 m evacuated beam path on the residual airpressure along the beam path.

FIG. 2 shows a first embodiment of a beam delivery system for an F₂laser emitting around 157 nm or another laser such as an ArF laser,emitting at less than 200 nm, including an enclosure providing an inertgas purged beam path.

FIG. 3 shows a dependence of the transmittance of a 157 nm beampropagating along a 0.5 m beam path purged with helium or nitrogen gason the number of flushings of the beam path using each of the two inertgases.

FIG. 4a shows a preferred embodiment of a beam delivery system for an F₂laser emitting around 157 nm or another laser such as an ArF laser,emitting at less than 200 nm, including an enclosure sealed with atransparent window providing an inert gas purged beam path.

FIG. 4b shows an experimental setup, and also an alternative embodimentwhen the detector P is replaced by an external housing including anoptical imaging system of a photolithography system and/or a workpiece.

FIG. 5 shows data of spectral absorption data for selected species.

FIG. 6 shows the effect of switching the laser off on the level of O₂along a purged beam path according to the preferred embodiment.

FIG. 7 shows the effect of switching the laser back on on the level ofO₂ along the purged beam path of the preferred embodiment.

FIG. 8 shows the rate at which an inert gas purged beam path returns tolow contamination level after flushing the beam path with O₂.

FIG. 9 shows the rate at which an inert gas purged beam path returns tolow contamination level after flushing the beam path with ambient air.

FIG. 10 shows a first pair of overlaying plots of O₂ concentration in apurged VUV laser beam path and laser power versus time, illustrating howthe O₂ concentration depends on the laser power.

FIG. 11 shows a second pair of overlaying plots of O₂ concentration in apurged VUV laser beam path and laser power versus time, illustrating howthe O₂ concentration depends on the laser power.

FIG. 12 shows plots of the dependence of oxygen concentration in theenclosure of FIG. 4b on the purge gas flow rate for argon and nitrogenpurge gases with and without the laser turned on.

FIG. 13 shows plots of the dependence of oxygen concentration in theenclosure of FIG. 4b on the laser power for nitrogen purge gas at 5.3 Wlaser power for various purge gas flow rates.

FIG. 14 shows plots of the dependence of generated oxygen concentrationin the enclosure of FIG. 4b on the purge gas flow rate for nitrogenpurge gas with the laser on.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a preferred embodiment of a beam delivery system for thepresent invention. The present invention may be used with any laser, butis particularly advantageous for a laser operating below 200 nm such asArF, Xe, F₂, Kr, Ar and H₂ lasers operating around 193 nm, 172 nm, 157nm, 145 nm, 125 nm and 121 nm, respectively. An F₂ laser systemoperating around 157 nm will be specifically referred to in thepreferred embodiment below. Resonator optics 1 are preferably mounted toa laser discharge chamber 2 or tube in such a manner that their tilt canbe adjusted, in order to align them to the optical axis of the resonator1. Preferred optical and electrical systems are described in greaterdetail in U.S. patent application Ser. Nos. 09/090,989 and 09/136,353and U.S. Provisional Application No. 60/120,218, each of which is herebyincorporated into the present application by reference. For example,means for selecting one of the closely-spaced natural emission linesaround 157 nm of the F₂ laser is part of the preferred optics.

A pair of main electrodes 3 is connected to an external power supplycircuit to supply a pulsed discharge to excite the molecular fluorine inthe gas mixture. In addition, UV-preionization of the electricaldischarge is also provided and may be realized by means of an array ofspark gaps or by another source of UV-radiation (surface, barrier orcorona gas discharges), disposed in the vicinity of at least one of themain electrodes 3 of the main discharge of the laser. A preferredpreionization unit is described in U.S. patent application Ser. No.09/247,887 which is also hereby incorporated by reference into thispresent application.

A housing or enclosure 4 containing the beam path is attached to anoutcoupling mirror holder 6 of the resonator optics 1 preferably throughvacuum bellows 8 and sealed with conventional o-rings (such as Viton™o-rings), flat packing or other sealing materials. This allows degreesof freedom necessary for optical alignment of the outcoupling mirror 6,while at the same time maintaining a vacuum-quality seal between theoutcoupler 6 and the beam path enclosure 4. The residual pressure withinthe beam path enclosure 4 preferably may be reduced to less than 200milliTorr, and specifically to 100 milliTorr or less.

The enclosure 4 is equipped with a purging gas inlet 10 and a gas outlet12 and means for controlling the gas flow rate, such as an adjustableneedle valve 14. If only one inlet 10/outlet 12 pair is used, the inlet10 and outlet 12 are spaced apart and preferably located at opposed endsof the enclosure 4. A long beam delivery system will preferably haveseveral pairs of gas inlets 10 and outlets 12. The inlets 10 and outlets12 are preferably positioned to provide a homogeneous medium within theenclosure along the beam path. In this way, every section of the beamdelivery system is sufficiently purged with low consumption of the purgegas. Even a short beam delivery system may have several gas inlets 10and outlets 12 especially, e.g., if a clear aperture within the beamdelivery system is blocked by built-in optical components and mounts.For example, the beam path may be partitioned with one or more opticalwindows.

The preferred vacuum level can be achieved by connecting a simple andinexpensive (e.g., 50 mTorr) one or two stage mechanical rotary vane orrotary piston pump or roughing pump (not shown) to the enclosure 4 via apump port 16. The pump port 16 need not be a separate connection to theenclosure 4. For example, the vacuum source may use the inlet 10 oroutlet 12 connection to the enclosure 4 which may be sealed off from thepump when the inert gas is flowing, such as by a T-valve or some similarcomponent.

Preferably, a 0.5 mbar 4-stage diaphragm pump is used. An oil vapor trapmay be used between the pump and the beam path enclosure, such as acryogenic trap or Micromaze [TM] filter. A three-stage diaphragm pump,which is relatively cheap and oil-free, can also be used. Alternatively,a more sophisticated pump or pumps may be used such as an oil diffusionpump, a cryogenic pump or a turbomolecular pump. The preferred“tightness” of the beam path enclosure 4 is equivalent to a leak rate ofone Torr-liter per minute or lower. The purging gas is preferablyultra-high purity (UHP) grade helium, argon, or neon, although otherinert gases (e.g., nitrogen) of UHP grade may also be used.

A preferred procedure of preparing the beam path enclosure 4 foroperation of a laser system of the present invention, and particularlyfor the F₂ laser emitting at 157 nm, is explained below. Note that thepreferred laser system includes a processor for controlling andcoordinating various components. The procedure for preparing the beampath, in accord with the present invention, may be manually- orprocessor-controlled. If a processor is used, vacuum gauge and gas flowmeter readings would be inputs. The processor would generate outputsignals for controlling the opening and closing of the pump port 16 andthe purging gas inlet(s) 10 and outlet(s) 12 and the flow control of thevalve 14.

The preferred method includes first, closing the gas inlet 10 and outlet12. Second, opening the pump port 16, and pumping down the enclosure 4with, e.g., a 50 milliTorr vacuum pump until the vacuum gauge indicatesthat a predetermined residual pressure has been reached within theenclosure 4, e.g., 100-200 milliTorr, or lower. In a preferredembodiment, the enclosure 4 is pumped down to around 0.5 Torr using a 3or 4 stage diaphragm pump. Next, the pump port 16 is closed off, theinlet port 10 is opened and the enclosure 4 is filled with inert gasflowing in through the inlet port 10 until approximately atmosphericpressure or higher is reached in the enclosure 4. Then, the inlet port10 is again closed and the pump port 16 opened to repeat the evacuationprocedure. These steps of evacuating the enclosure 4 followed byback-filling the enclosure 4 with inert gas are preferably repeatedseveral times.

After these several gas flushing cycles, the pump port 16 is closed andboth the gas inlet 10 and gas outlet 12 are opened. A gas flow at aselected flow rate, preferably around 0.1 liters per minute, isestablished and maintained in the enclosure 4 through control of theflow control valve 14. The pressure is now maintained around atmosphericpressure or preferably slightly higher. The beam path enclosure is nowready for working operation of the laser.

FIG. 3 shows that the transmittance of a 157 nm beam from an F₂ laseralong a 0.5 meter long optical path using helium and nitrogen asflushing gases. The transmittance is shown as increasing with the numberof flushes, but becomes asymptotic to its highest value in as few aseight (8) “flushing” cycles. Of course, more than eight flushing cyclesmay nonetheless be used. As can be seen, for helium, close to 99%transmittance can be achieved with eight flushes. The results usingnitrogen were not as good as with helium. However, the nitrogen used inthe experiments has a specified level of water of only 3 ppm, while UHPhelium was much more pure and had a specified water level of less than 1ppm which may have accounted for the difference in performance.

The present invention teaches that using cycles of evacuating andfilling the enclosure 4 with inert gas allows drastically reducedpreparation times and also minimizes inert gas consumption. After theseflushing cycles are performed, a preferred flow rate of 0.1 liters perminute is sufficient to maintain high transmittance for a substantialperiod of time. The entire preparation cycle advantageously takes only afew minutes. In addition, relatively inexpensive pumps and lower costsealing arrangements can be used.

In another aspect of the invention, it is recognized in the presentinvention that contaminants may be generated within the enclosure whichmay flow onto a workpiece that is being processed or exposed using theVUV laser beam that is traversing the interior of the enclosure, or ontooptical equipment. The contaminant generation rate is recognized asbeing related to the operation of the laser, such as due to theinteraction of the VUV beam or stray light therefrom with componentswithin the enclosure or the enclosure itself. It is further recognizedin the invention that these contaminants may conventionally flow out ofan opening at the end of the enclosure (see FIG. 2).

In accordance with the present invention, then, referring to FIG. 4a, aVUV transparent window 18 is provided to seal the interior of theenclosure 4 from a workpiece 20, or other processing or beam shapingequipment, e.g., an optical imaging system, that the VUV beam 22 may bedirected towards. The window 18 is transparent to VUV light, and so thewindow is made of preferably CaF₂ and alternatively a material such asBaF₂, LiF, SrF₂, MgF₂, quartz and fluorine doped quartz, or anothermaterial that may be known to one skilled in the art as beingsubstantially transparent to light around 157 nm. The window may have anantireflection coating on it, as well. Thus, the VUV beam is allowed toescape the enclosure which protects the beam from attenuation, whilecontaminants generated within the enclosure are unable to escape and arethereby prevented from deteriorating a workpiece 20 or other processingor optical equipment.

The concerns addressed by this aspect of the invention wherein a windowis included at the end of the beam path of the molecular fluorine VUVlaser system, are shown in more detail below as being verified byexperimental results. The presence of the VUV beam is shownexperimentally as providing an increase in the contamination level inthe enclosure. For example, when a copper enclosure was used, acontamination level of O₂ was measured to be 0.5 ppm when the laser wasoff, and 0.8 ppm when the laser was on under otherwise identicalconditions. A laser running continuously for two days and having astainless steel enclosure was shown to have an O₂ contamination levelbetween 0.4 and 0.5 ppm, which fell to between 0.25 and 0.3 ppm when thelaser was switched off. The O₂ level increased back to between 0.45 and0.55 ppm when the laser was later switched back on.

Thus, particularly when the laser is running, it is advantageous to havethe window of the present invention to block impurities from exiting theenclosure and deteriorating a workpiece 20 or other processing equipmentoutside the enclosure 4. The workpiece 20 or other processing equipmentsuch as optical imaging equipment, etc., may be protected by its ownenclosure (not shown) which protects the workpiece 20, etc., fromcontaminants such as O₂, H₂O, hydrocarbons and dust in the atmosphere.This external enclosure (not shown) for the workpiece may be a cleanroom or a smaller housing. The external enclosure (not shown) may besealably connected to the enclosure 4 at the window 18, whereby thewindow 18 seals the enclosure 4 from the enclosure (not shown). Thus,the enclosure 4 and the external enclosure (not shown) are optically andmechanically coupled together, although there is not fluid communicationbetween the two enclosures. The window 18 advantageously preventscontaminants generated in the VUV laser enclosure 4 from entering theexternal enclosure (not shown) for an imaging system and/or workpiece.

The window 18 itself may be kept clean by using a method of flowing veryclean gas past the window to prevent contaminated gas from accessing thewindow and depositing a film that might absorb VUV light and attenuatethe beam. The technique set forth in U.S. Pat. No. 4,534,034 (herebyincorporated by reference), whereby an electrostatic precipitator isused to clean some portion of gas before flowing that gas to a lasertube window, may be used to keep the window 18 clean. In addition, a setof baffles and/or a precipitator may be positioned near the window 18 totrap contaminants and keep them from accessing the window 18.

It is recognized in the invention that the generated contaminants mayalso deteriorate the atmosphere within the enclosure such as toattenuate the VUV beam notwithstanding that the method of pumping andpurging the interior of the enclosure with an inert gas as describedabove is in place. It is further recognized in the invention that thedegree of contamination generated within the enclosure may depend on thematerials that the enclosure is made of. Moreover, the particularpressure within the enclosure may effect the performance of the system.Lastly, the particular purge gas being used may enhance or reduce theperformance or the benefits of the enclosure according to the aboveaspect of the invention. These features are not only recognized in thepresent invention as potentially effecting the system, but as describedbelow, advantageous beam enclosures are provided in accordance with thatrecognition and in accordance with the preferred embodiment.

Experiments were performed in accordance with the features recognized aseffecting the molecular fluorine laser system and beam path enclosure 4(see above). In accordance with the present invention, a system isprovided that is improved based on results produced in experimentsconducted with respect to that recognition.

FIG. 4b schematically illustrates the experimental setup used. FIG. 4balso illustrates alternative features to the embodiment shown at FIG.4a, including an enclosure sealably connected to an F₂ laser. Theenclosure 24 has an inlet port 26 and an outlet port 28 for flowing theinert gas through the enclosure, and vacuum bellows at either end of theenclosure 24 to facilitate connection to the laser and to aphotodetector P. The inlet port 26 has an adjustable valve V foradjusting the flow rate of the gas purge. An additional valve may beincluded for connecting the enclosure 24 to an evacuation port eitherthrough the inlet or outlet ports 26, 28 or through an additional port 9not shown). The outlet port is connected to a moisture contentmonitoring system 30 and an O₂ monitoring system 32.

The preferred system for application processing would include a window18 as described above with respect to FIG. 4a and the photodetectorwould be replaced by an application process such as an imaging systemand workpiece or only a workpiece, wherein the imaging system and/orworkpiece would typically be in a housing sealably connected to theenclosure 24, just as the photodetector is shown in FIG. 4b to besealably connected to the enclosure 24.

Long term exposure tests were carried out using a photodetector Pincluding pre-production-type SXUV -and PtSi-photodiodes. In this way,the output energy of the VUV beam could be accurately measured so thateffect on that energy by changing materials, such as of the enclosureitself or of the purging gas supply, and other parameters, such as thepressure, of the interior of the enclosure could be noted. In additionto monitoring the output energy of the VUV beam, the experiments werealso carried out with a separate, continuous monitoring of the O₂ andwater vapor content within the purged exposure box 24 using the H₂Omonitor 30 and O₂ monitor 32. Purge gases used were argon and nitrogen.The flow rate was varied between 10-300 l/h. The laser power was variedbetween 0-10 W. In addition, the material composition of the enclosure24 was varied (copper, stainless steel and PTFE hoses were used).

Some preferred equipment for carrying out the experiments on theincrease or decrease of O₂ and H₂O-vapor density in the enclosure 24under predetermined conditions are included below:

O₂ detection system (32): model DF153-100 from Delta F corp.

Moisture Analyzer (30): model 1C-C1 DewTrace from Edge Tech

Both analyzing systems 30 and 32 are identical to those used at MITLincoln Lab and operate well in a detection range of 0.1 ppm to 100 ppmfor each of the contaminants measured.

Experimental set up: The exposure box 24 was purged with a flow rate ofV=90 liters/hour, against open air with an estimated overpressure of <50mbar. This range between zero and 50 mbar overpressure was recognized inthe present invention, and verified in the experiments as being therange of pressures that provide optimal results. Three additional energymonitors and six energy detectors are illuminated in a long term run at1 kHz. The beam fluence on the optics was F=10 mJ/cm², and on thedetectors 10 μJ/cm² or 10 μJ/cm² depending on specific position of theoptics and the detectors. The laser was operated in Energy—constantmode, whereby power was checked by the multiple energy monitors(reading) and by a LM100E Coherent power meter. The laser power wasvaried by varying the repetition rate of the laser.

Some background information:

Jenoptik LOS has summarized the absorption cross sections of the variouscontaminants at 157 nm (see Table 1). Other references review theabsorption of these molecules in a wider range of wavelengths (see FIG.5). The absorption coefficients of O₂, water vapor, and N₂ are,respectively, 140 cm⁻¹, 64 cm⁻¹, <0.0002 cm⁻¹. The general contaminationlevels which are in discussion as being desired for the various stagesof a lithography system including a VUV or ArF laser in accord with thepresent invention cover a range between less than 1 ppm and more than100 ppm (referring to most clean optics regions and open end waferstages). A purity of the N₂ purge gas of a grade 7.0 or even 9.0 ispreferred, although grade 5.0 purity N₂ gas may be sufficient dependingon other system conditions.

Effect of N₂ purge gas delivery:

A first identified result of this investigation pursuant to therecognition in the present invention that materials making up theenclosure may effect the contamination level in the enclosure revealsthat PTFE tubing is not desirable for use as a material for theenclosure 4.

A Brief Summary Follows

The contamination level of O₂ for:

(a) stainless steel tubing was 0.3 ppm (with the VUV laser beam turnedoff);

(b) purge through copper tube, output of exposure box was 0.5 ppm (withthe VUV laser beam turned off);

(c) purge through copper tube, output of exposure box, was 0.8 ppm (withthe VUV laser beam turned on)

(d) purge through PTFE-tubes, output of exposure box, 3.5 ppm (with theVUV laser beam turned on)

(e) additional 4 m MFA- gas delivery hose inserted (all other conditionsthe same) ˜8 ppm (with the VUV laser beam turned on).

These findings reveal that the preferred housing 24 is made of stainlesssteel or copper, and that PTFE and MFA hosing is not desired. Thecontamination level, and resulting attenuation of the VUV beam, issubstantially higher when PTFE hoses are used as opposed to when astainless steel or copper housing is used. Another material such asglass could be used for achieving low contamination levels comparablewith those achieved using copper or stainless, but glass is notpreferred for other considerations such as handling practicalities inthese systems.

These findings quantitatively confirm another recognition of the presentinvention that when the VUV laser beam is turned on, the contaminationlevel is higher than when the beam is off. It is recognized in theinvention, and the experiments showed, that H₂O vapor/moisturecontamination levels were relatively uneffected by the different setupsof tubing or housing materials, or by the laser operation conditions,i.e., whether the VUV beam was turned on or off.

Repeatable experiments revealed some interesting observations regardingthe following typical behavior with respect to the time constant rate ofchange of the contamination level after opening the exposure box:

O₂ H₂O Exposure box closed, 1 pump- 1.2 ppm 2.0 ppm flush w/ N₂ cycle,start Laser on, increase in O₂ 2.0 ppm 1.9 ppm contamination, after 30min.: Lasers keeps running continuously, 1.3 ppm 1.6 ppm e.g., after 2.5h: Laser continuously on, after longer 0.5 ppm 0.8 ppm time, e.g., 15 hThese levels thereafter, ˜0.4-0.5 ppm ˜0.8-0.9 ppm e.g., after 2 days:

If laser is switched off, but exposure chamber is continued to be purgedw/o interrupt, the O₂ contamination level drops fast down to, about0.25-0.30 ppm If after a pause the laser is switched on again, the O₂concentration rises again to about 0.45-0.55 ppm. This behavior isillustrated by the experimental results shown at FIGS. 6 and 7.

Another interesting observation shows the decrease of the O₂ level afterflushing the exposure box with normal air and with pure O₂, both up tonormal pressure. The decrease of O₂ contamination occurs with twodifferent time constants from the 1 ppm level (1 ppm =full scale of theplotter and v=3 cm/h), as illustrated by the experimental results shownat FIGS. 8 and 9. Thus, after about 4.3 Million laser pulses or shots,as shown in FIG. 8, a constant low O₂ level in the case of pure O₂flushing is again achieved. After more than 25 Million pulses, as shownin FIG. 9, the low constant level is achieved when the box is insteadflushed with ambient air.

FIGS. 10 and 11 each show overlayed plots of O₂ concentration within thepurged beam path enclosure of the preferred embodiment and laser poweras a function of time. Each of FIGS. 10 and 11 illustrate how the O₂concentration depends on the laser power.

The beam path enclosure of FIG. 10 was purged with 99.999% purity N₂gas, or “5 grade” N₂ gas. The lowest O₂ concentration was observed whenthe laser was turned off (A). The O₂ concentration is shown to increasewith increasing laser power from about 0.2 ppm when the laser is turnedoff to about 0.5 ppm when the laser is at full power (around 10 W).

It is recognized in the present invention that O₂ is likely produced bydissociation of residual H₂O content in the non-perfect N₂ gas purge.Thus, high purity inert purge gas is preferred. The inert gas may be anoble gas or nitrogen or another gas that does not absorb VUV radiation,and is preferably N₂, He, Ne, Kr or Ar. The purity of the inert purgegas is preferably greater than 99.5% purity. Even more preferred is ahigher grade purity N₂ gas, such as at least 99.9% purity or more. The 5grade purity nitrogen gas, i.e., 99.999% purity, used in the experimentsis an example. In addition, “7 grade” or 99.99999% pure N₂ gas may beadvantageously used for reducing the O₂ concentration in the purged beampath enclosure of the preferred embodiment. Still greater purity inertpurge gas such as 9 grade, or 99.9999999% purity gas would result in alower O₂ concentration in the enclosure. These purities may also be usedfor another inert gas such as He, Ne, Kr or Ar.

The beam path enclosure of FIG. 11 was purged with N₂ gas at flow ratesof around 150 liters/hour and 300 liters/hour, demonstrating that the O₂concentration reduces with increased N₂ flow rate. The laser wasoperated in energy constant mode and the laser power was varied byvarying the repetition rate from 100 Hz to 1000 Hz. The O₂ concentrationwas observed to decrease with decreasing laser power.

FIGS. 12-14 confirm the recognition in the present invention that thegas flow rate, inert gas used and laser power each affect theconcentration of oxygen in the enclosure 24. FIG. 12 shows plots of O₂concentration versus purge flow rate for argon purge gas with the laseron and off, and for nitrogen purge gas with the laser on and off. Theexperiments showed that the oxygen concentration reduces sharply withflow rate up to around 150 l/h, and decreases more gradually from 150l/h to 300 l/h for both gases with the laser on or off. The nitrogenpurge gas yielded lower oxygen concentrations than the argon purge gasunder the same laser operation conditions. The oxygen concentration wasgreatly reduced when the laser was turned off compared with when thelaser was turned on.

FIG. 13 shows the dependence of the oxygen concentration in theenclosure 24 on the laser power at varying nitrogen purge gas flow ratesof 90 l/h, 150 l/h and 270 l/h. Higher oxygen concentrations wereobserved at lower flow rates. Although the oxygen concentrationincreased with laser power at each flow rate, the oxygen concentrationincreased more rapidly with laser power at lower flow rate. For example,at 270 l/h, the oxygen concentration is barely observed to increase from0-10 W laser power, remaining around 0.2 ppm, whereas at 90 l/h, theoxygen concentration increased from around 0.4 ppm to around 1.0 ppmfrom 0-10 W laser power.

FIG. 14 shows the generated O₂ concentration versus nitrogen purge flowrate with the laser turned on and operating at around 5.4 W. The oxygenconcentration is observed to decrease substantially asymptotically fromaround 0.6 ppm at 50 l/h to around 0.05 ppm at 300 l/h, wherein the plotis of the estimated oxygen concentration generated due to the presenceof the VUV laser beam in the enclosure, instead of the total O₂concentration.

In brief, a contamination level of less than 1 ppm can be achieved forboth O₂ and H₂O in a purging enclosure 4 such as that described abovewherein the purging gas is 5.0 grade N₂. There is no tendency observedof approaching the Zero-light-level, or laser off level, even when thelaser is running for several days. The VWV-radiation itself appears toincrease the O₂ content in the outlet of the enclosure. It may bespeculated that the reason for this is one or more of the following:

(a) a cracking of residual H₂O content in the N₂ purge gas; and

(b) outgassing from mirrors and/or beam splitter surfaces or from thewalls of the enclosure 4 (but this should be run in an asymptoticdecrease of the O₂ content, too).

As described above and at FIGS. 6 and 7, it is observed that there is anincrease of the O₂ contamination level when the laser radiation is onversus when the laser is turned off.

It is therefore advantageous to have the window 18 shown at FIG. 4 anddescribed above to separate the enclosure 4 of the molecular fluorinelaser from the purge volume of external processing equipment and/or theworkpiece that the VUV beam is directed to. Otherwise, contaminationwhich arises due to the VUV radiation in the enclosure 4 may contaminatethe purge volume of the external equipment or workpiece at anundesirable or intolerable level.

In addition, the experiments showed that higher O₂ contamination levelsoccur when PTFE-hoses are used in the enclosure 4 of the laser purge gasline versus using stainless steel and/or copper for the material of theenclosure. Thus, advantageously, the enclosure of the present inventionuses stainless steel and/or copper for the material of the enclosure.

The present invention can be applied as well to an enclosure for a beamline for other radiation below 200 nm, such as is affected by absorptionin O₂ and H₂O. Examples include the 193 nm output emission of the ArFexcimer laser, or a frequency multiplied output of a solid state laseror dye laser. That is, a fluctuation in O₂ will effect the amount ofabsorption occurring in a 193 nm beam line, or another sub-200 nm beamline, and so the present invention may be advantageously applied to theArF laser, or another laser emitting under 200 nm, as well as to themolecular fluorine laser.

The above description is not meant to set forth or limit in any way thescope of the present invention, but only to provide examples ofpreferred and alternative embodiments. Instead, the scope of the presentinvention is that set forth in the claims that follow, and structuraland functional equivalents thereof.

For example, what is described at any of U.S. Pat. Nos. 6,005,880 and6,002,697, and U.S. patent application Ser. Nos. 09/317,695, 09/130,277,09/172,805, 09/379,034, 09/244,554, 09/317,527, 09/327,526, 09/447,882,60/162,845, 09/453,670, 60/122,145, 60/140,531, 60/166,952, 60/173,993,60/166,277 and 60/140,530, each of which is assigned to the sameassignee and is hereby incorporated by reference, may be practiced incombination with what is described above and below.

What is claimed is:
 1. A system for providing sub-200 nm exposureradiation for lithography, comprising: a lithographic exposure radiationgenerating source for outputting a beam at a wavelength less thansubstantially 200 nm; and an enclosure for separating a beam path forthe exposure radiation exiting the exposure radiation generating sourcefrom the outer atmosphere, and for removing sub-200 nm photoabsorbingspecies therefrom, such that the energy of the beam can traverse saidenclosure without substantial attenuation due to the presence of sub-200nm photoabsorbing species along said beam path.
 2. The system of claim1, wherein said enclosure means contains no more than 0.5 ppm O₂.
 3. Thesystem of claim 1, wherein the lithographic exposure radiationgenerating source comprises a laser system including a discharge chamberfiled with a gas mixture, a plurality of electrodes within the dischargechamber coupled with a power supply circuit and a resonator.
 4. A systemof claim 1, further comprising one or more ports for evacuating theenclosure and removing sub-200 nm photoabsorbing species therefrom andmaintaining said beam path substantially free of sub-200 nmphotoabsorbing species to enable the beam substantially to propagatealong said beam path.
 5. The system of claim 1, wherein said enclosuresubstantially comprises one or more materials selected from the group ofmaterials consisting of stainless steel and copper.
 6. The system ofclaim 1, wherein said enclosure is sealably coupled to said lithographicexposure radiation source.
 7. The system of claim 1, in which the beamis provided by a laser selected from the group consisting of a F₂ laserand an ArF laser.
 8. The system of claim 1, further comprising at leastone optical component within the enclosure, and wherein said exposureradiation interacts with said at least one optical component within saidenclosure that is protected from being substantially disturbed by saidcontaminant species.
 9. A system for providing sub-200 nm exposureradiation for lithography, comprising: a lithographic exposure radiationgenerating source for outputting a beam at a wavelength less thansubstantially 200 nm; and an enclosure for separating a beam path forthe exposure radiation exiting the exposure radiation generating sourcefrom the outer atmosphere, and for flowing an inert gas therein having apurity in a range between 99.5% purity and 99.9999999%, such that theenergy of the beam can traverse said enclosure without substantialattenuation due to the presence of sub-200 nm photoabsorbing speciesalong said beam path.
 10. The system of claim 9, wherein the purity ofsaid inert gas is between 99.9% and 99.9999999%.
 11. The system of claim9, wherein the purity of said inert gas is between 99.999% and99.9999999%.
 12. The system of claim 9, wherein said enclosure meanscontains no more than 0.5 ppm O₂.
 13. The system of claim 9, wherein thelithographic exposure radiation generating source comprises a lasersystem including a discharge chamber filed with a gas mixture, aplurality of electrodes within the discharge chamber coupled with apower supply circuit and a resonator.
 14. The system of claim 9, furthercomprising one or more ports for evacuating the enclosure and flowingthe inert gas within said enclosure and maintaining said beam pathsubstantially free of sub-200 nm photoabsorbing species to enable thebeam to propagate along said beam path.
 15. The system of claim 9,wherein said inert gas is selected from the group of gases includingnitrogen, argon, neon, krypton and helium.
 16. The system of claim 9,wherein the purity of said inert gas is in a range between 99.99999% andsubstantially 99.9999999%.
 17. The system of claim 9, wherein said inertgas is flowed at a flow rate of at least 150 liters per hour.
 18. Thesystem of claim 9, wherein said inert gas is flowed at a flow rate ofless than substantially 0.2 liters per minute.
 19. The system of claim9, wherein said enclosure is maintained at an overpressure of less thansubstantially 50 mbar.
 20. The system of claim 9, wherein said enclosuresubstantially comprises one or more materials selected from the group ofmaterials consisting of stainless steel and copper.
 21. The system ofclaim 9, further comprising at least one optical component within theenclosure, and wherein said exposure radiation interacts with said atleast one optical component within said enclosure that is protected frombeing substantially disturbed by said contaminant species.